A power supply device including an assembled battery configured of a plurality of battery cells includes a status-monitoring unit for monitoring a status of the battery cells in order to avoid abnormal states such as over-charge and over-discharge of the battery cells. The status-monitoring unit detects data such as a voltage, an electric current, and a temperature from each battery cell as needed, and monitors a status of the battery cell based on the detected data. In particular, since a lithium ion battery can be used in a wide range of SOC (state of charge), an over-discharge region, an overcharge region, and a normal region are disposed near to each other. Therefore, it is preferable that voltage management is carried out more strictly than in the other types of batteries.
Furthermore, it is known that the states of charge of respective battery cells constituting the assembled battery become nonuniform due to variation in charge and discharge or temperature of battery cells. When the states of charges of the battery cells constituting the assembled battery become nonuniform, it is not possible to carry out efficient charge and discharge capable of making the most of the function as the assembled battery. In order to solve such problems, a power supply device provided with a resistance discharge-type discharge circuit that equalizes energy has been proposed (Patent Literature 1).
The discharge circuit according to Patent Literature 1 is connected in parallel to each battery cell, and includes a switch and a discharge resistor. A working status of the switch is controlled by a switch control section. The switch control section is included in the status-monitoring unit. The switch control section is configured to control a working status of the switch depending on a status of the battery cell detected by the status-monitoring unit and to discharge any battery cell. Thus, the state of charge can be adjusted.
In the power supply device of Patent Literature 1, the status-monitoring unit is built in an integrated circuit. The integrated circuit is provided with a terminal to which a measurement line for detecting a voltage is connected, and a terminal to which a signal line for controlling the discharge circuit is connected. As the number of electrical components to be built in the integrated circuit is increased, the number of electrical components to be mounted on the circuit board can be reduced. This configuration contributes to downsizing of a circuit board and cost down.
Furthermore, the power supply device according to Patent Literature 1 includes a first voltage detection circuit and a second voltage detection circuit in order to provide redundancy to the voltage detection of a battery cell. This configuration enables a voltage of one battery cell to be detected by two voltage detection circuits. For example, even if one of the detection circuits fails, the other detection circuit can detect a voltage of the battery cell.